a(1)=1, a(n) = a(n-1) + (p-1)*p^(n/2-1) if n is even, else a(n) = a(n-1) + p^((n-1)/2), where p=4.

Terms

a(0) =1a(1) =4a(2) =8a(3) =20a(4) =36a(5) =84a(6) =148a(7) =340a(8) =596a(9) =1364a(10) =2388a(11) =5460a(12) =9556a(13) =21844a(14) =38228a(15) =87380a(16) =152916a(17) =349524a(18) =611668a(19) =1398100a(20) =2446676a(21) =5592404a(22) =9786708a(23) =22369620a(24) =39146836a(25) =89478484a(26) =156587348a(27) =357913940a(28) =626349396a(29) =1431655764